The present invention relates to identifying and mapping subsurface fluid flow structure which may be particularly useful in analyzing the evolution of underground petroleum and natural gas reserves in large scale basins and small scale reservoirs, and their migration and drainage networks. Such information is of great interest to the petroleum industry and may be used to site wells for maximum recovery.
Most major hydrocarbon producing basins of the world have deep stratigraphic layering and structures that may be mapped using well log data and/or seismic imaging. Hydrocarbon reservoirs are embedded within such stratigraphic layers. These hydrocarbon reservoirs must be interconnected by migration pathways that lead to fluid sources deep within the earth. Further, the individual reservoirs experiencing production must have finescale drainage structure over which short term variations (e.g., 1-10 years) in hydrocarbon signals should be observable.
At the large scale (entire sedimentary basin regions) it would be useful to identify the interconnectivity between hydrocarbon reservoirs and migration pathways. At the small scale (individual producing reservoirs) it would be useful to identify high porosity drainage structure, regions of bypassed pay and gas-oil-water volumes. Robust methodology is required to identify these structures both at the basin scale and at the reservoir scale such that variations in multiple seismic surveys may be exploited and the derived geometry of the regions of interest will be stable among surveys.
In the prior art, two dimensional (2-D) seismic reflection profiles and three dimensional (3-D) seismic surveys have been used to predict the location of oil and gas reservoirs and to site wells. Typically, well-dependent log analysis techniques then locate the specific production intervals within each well. Seismic surveying produces geological imaging of the subsurface determined from the acoustic signature recorded at the top surface of a volume of earth. A typical seismic survey covers a volume of several tens of cubic kilometers. In 3-D seismic surveys, the geological formations and structures within the earth are determined from overlapping energy ensonifications (acoustic reflections) within that volume. The basics of seismic analysis may be found, for example, in "Simple Seismics," N. A. Anstey, International Human Resources Development Corp., 1982.
Previous work in identifying hydrocarbon reservoirs from seismic surveys have employed seismic attribute analysis to identify subsurface features consistent with the presence of oil or gas. Seismic attribute analysis typically involves complex-valued signal analysis of seismic waveforms. The seismic features consistent with the presence of petroleum are often referred to as "Bright Spots," and are herein referred to as "High Amplitude Regions." For example, U.S. Pat. No. 4,479,204 to Silverman discusses the association of Bright Spots with subsurface oil or gas reservoirs.
High Amplitude Regions may be identified using a variety of seismic attributes. It is well known, for example, that complex trace analysis may be performed on reflection seismic traces to derive reflection strength, instantaneous phase, instantaneous amplitude and other seismic attributes. An exemplary discussion of this analysis may be found in M. T. Taner et al., "Complex Seismic Trace Analysis," Geophysics, Vol. 44, No. 6, pp. 1041-63 (June 1979). These seismic attributes can often be associated with the presence of oil or gas deposits. For example, a change in the reflection strength and/or instantaneous phase attributes is often observed across the top of oil or gas reservoirs.
Techniques as discussed in Taner et al. for using the complex trace analysis on seismic reflection traces and examining the attributes of such traces, have the shortcoming of not providing reliable mapping of drainage features within individual reservoirs. The trace attributes utilized by Taner et al. have high frequency components that may not effectively identify the physical structure of interest. Small scale structure should not be inferred from the high frequency components because extraction of any such information is not justified by the wavelength of seismic energy, typically about 75 feet.
It has been found that the use of smoothed or envelope information of the seismic traces yields more reliable information regarding true physical structure. As described in U.S. Pat. No. 5,311,484 to Anderson and He (hereinafter "Anderson and He"), incorporated in full herein by reference, the geopressure transition zone in the Gulf Coast of offshore Louisiana, for example, is between 100 and 1,000 feet thick, and the trend of the transition zone may be more precisely identified by use of a smoothed reflection strength trace. Complex trace analysis is preformed on the instantaneous amplitude trace to obtain the "second reflection strength," which is effectively the envelope of the instantaneous amplitude utilized by Taner et al. This second reflection strength has been found to better map top-of-geopressure surfaces, as in Anderson and He, in that broad trends may be identified. The second reflection strength is also a valuable seismic attribute for identifying regions containing hydrocarbons in that spurious high frequency data is eliminated due to the processing of the trace in calculating the second reflection strength.
Significantly, hydrocarbon reservoirs are not acoustically static over their history due to production therefrom or natural drainage processes. The location and content of oil or gas reservoirs changes substantially due to natural drainage and production of water, oil and gas. Additionally, the seismic attribute imaging of the prior art is highly sensitive to the methodology and processing involved in each seismic survey, and thus even large scale static features cannot be faithfully reproduced over two or more spatially overlapping surveys. Accordingly, the prior art which processes single surveys is incapable of accurately tracking and identifying the interconnectivity and drainage history of regions containing hydrocarbons. Multiple 3-D seismic surveys taken at different times, or four dimensional (4-D) surveys, over both large scale basins and within small scale reservoirs can yield vital information for assessing prospective well target locations in hydrocarbon prospecting, by better defining large scale structure and allowing for the analysis of changes in small scale regions by differencing 3-D data sets.
It is expected that fine scale drainage structure within reservoirs should be observable upon differencing seismic data taken at different times of the High Amplitude Regions of a reservoir of interest. For several years, laboratory measurements of the changes in acoustic reflection coefficients caused by changes in oil, gas, water and effective pressure have been predicting that field monitoring of enhanced recovery processes such as water and stream floods should produce mappable acoustic differences over time within a given reservoir. See, e.g., Nur, "Seismic Imaging in Enhanced Recovery," SPE Paper 10680, 1982. Successful field monitoring of oil/water boundary movements during waterfloods have been carried out in several fields worldwide (see, Breitenbach et al., "The Range of Applications of Reservoir Monitoring," SPE Paper 19853, 1989; and Dunlop et al., "Monitoring of Oil/Water Fronts by Direct Measurements," SPE Paper 18271, 1988) utilizing the physical principles determined in the laboratory. In locations such as the Gulf of Mexico, where the impedance contrasts of lithologies versus fluid mixes are sufficiently large, 4-D (multiple 3-D surveys over time) seismic monitoring can define drainage and gas-oil-water boundary movements. Such analysis has not been accomplished in the prior art.
Two major seismic amplitude changes can occur over time periods of interest (i.e., 1-10 years between surveys) in hydrocarbon producing reservoirs. These changes work in tandem with a general decrease in seismic amplitude that occurs due to the increase in differential pressure (lithostatic minus formation fluid pressure) as hydrocarbons are drained from a reservoir. One change is known as a "dim-out," which is caused by the replacement of oil with water. This causes a marked decrease in seismic amplitude which augments the overall decrease in seismic amplitude due to increased differential pressure. The second change is caused by the formation of a gas cap which occurs when gas comes out of solution from the oil as reservoir pressure drops upon production. A gas cap results in a very marked increase in seismic amplitude which overcomes the decrease that occurs due to increased differential pressure. Accordingly, despite the overall trend in decreasing seismic amplitude that occurs due to increased differential pressure caused by production, the dim-out and gas cap phenomena are believed to allow the segregation of oil, water and gas seismic signals and, thus, allow identification of fluid boundaries and drainage within a High Amplitude Region within a reservoir. It may be expected that within a reservoir of interest, differenced survey data of the hydrocarbon bearing reservoir from two or more periods should show clearly segregated regions of water, oil and gas, including drainage structure. High porosity regions with bypassed oil and gas (pay) which experience little change in differential pressure, can be identified as regions of near zero change in amplitude within a High Amplitude Region over time. These areas of high porosity may correlate with the drainage pathways within a reservoir which will be highly attractive well sites for the future discovery of bypassed pay. Such pathways have never been seismically imaged by the prior art.